Phosphatic materials and methods for the production thereof



July 18, 1961 P. D. MANNING ETAL 2,992,914

PHOSPHATIC MATERIALS AND METHODS FOR THE PRODUCTION THEREOF OriginalFiled May 27, 1955 2 Sheets-Sheet 1 July 18, 1961 p PHOSPHATIC M THEPRODUCTION THEREOF Original Filed May 27, 1955 MANNING ETAL 2,992,914

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United States 1 Claim. 01. 71-40 This invention relates generally to theproduction of various phosphatic materials from phosphate rock. Moreparticularly, the invention relates to the production from phosphaterock of feed grade dicalcium phosphate and a high analysis phosphatefertilizer product.

There has developed in recent years a substantial market for dicalciumphosphate of a grade and quality useful as an animal feed supplement. Insuch feed grade dicalcium phosphate, the weight ratio of phosphoruscalculated as elemental phosphorus to fluorine calculated as elementalfluorine must be not less than about 100, and preferably not less thanabout 200. Inasmuch as phosphate rock contains in the natural statesignificant amounts of fluorine, the production of feed grade dicalciumphosphate from phosphate rock has presented a particularly diflicultproblem. This invention, accordingly, embraces a process for themanufacture of feed grade dicalcium phosphate from phosphate rock, whichprocess is particularly advantageous in that it may be integrated on acommercial scale with the production of a high analysis phosphatefertilizer.

Since a substantial portion of the cost of fertilizer must defraytransportation expenses, it is frequently economically unsound for thefarmer to purchase low analysis fertilizer. Fertilizer manufacturers,therefore, generally concurrently with the development of the feed gradedicalcium market have been required to produce increasingly largepercentages of high analysis phosphate fertilizer products. Theconventional high analysis phosphate fertilizer product of the prior artis triple superphosphate in which substantially the entire phosphoruspentoxide content is watersoluble monocalcium phosphate. The addition ofsuch material to the soil at the time of planting gives rise to thecondition wherein the young plants have available, for immediate use,large quantities of phosphorus pentoxide, while the same plants at theirlater stages of development have available less phosphorus pentoxidebecause of the leaching of such values from the soil by rain andirrigation waters. Studies by Nelson et al., Soil Science SocietyProceedings, vol. 12, pages 1l3-ll8 (1948), using superphosphatecontaining radioactive phosphorus show that in the case of manyfertilized crops, particularly corn, the phosphate is required inlargest degree by the plant as it approaches maturity, and to a lesserdegree while the plant is in the seedling stage. These studies have alsoshown that plants approaching maturity have been forced to piratephosphorus values from the permanent soil structure because of theabsence of the phosphorus derived from the applied fertilizeroccasioned, at least in part, by previous leaching of phosphorus valuesfrom the applied fertilizer while these same plants were undergoingtheir initial growth periods. Thus, the application of water-solublephosphate values to the soil at the time of planting results in largequantities of phosphorus being present at a time when these largequantitles are not needed, and fewer of such values from the fertilizerbeing present at a subsequent date when large amounts are needed. Thisinvention accordingly is in part concerned with the production fromphosphate rock of novel, high analysis phosphate fertilizer whichcontain a substantial portion of available water-insoluble atentphosphate values and which, therefore, are effective to supplyfertilizing values to plants throughout the normal growing season. Thehigh analysis phosphate fertilizer products which the invention embracescan be ammoniated or otherwise modified to provide a more complete highanalysis fertilizer. I

It is accordingly a primary object of the invention to provide a methodfor the production from phosphate rock of feed grade dicalciumphosphate.

It is more particularly an object of the invention to pro vide a processfor the calcium defluorination of aqueous solutions offluorine-containing phosphatic materials pursuant to which the extent ofloss of phosphate values to the defluorination procedure is minimized.

It is an additional object of the invention to provide a method forcontrolling and minimizing a percentage of the phosphate values presentin the phosphate rock treated which are converted into phosphaticfertilizer and to thereby insure a maximum yield of the more valuabledicalcium phosphate.

It is an additional primary object of the invention to provide a methodfor the production from phosphate rock of a high analysis phosphatefertilizer.

It is a further object of the invention to provide a high analysisphosphate fertilizer product effective to provide phosphate values toplants throughout a normal annual growing season.

It is a specific object of the invention to provide a method for theproduction of a novel, multi-layered, pelletized phosphate fertilizerproduct.

It is a more specific object of the invention to provide an integratedcommercialprocess for the concurrent production of a novel high analysisphosphate fertilizer and of feed grade dicalcium phosphate.

The invention generally finds utility in conjunction with processeswhich entail acidulation of phosphate rock, extraction of thesolubilized phosphate values of the acidulated rock with an aqueousmedium, and processing the phosphate-rich extract so obtained to producea high analysis fertilizer or a feed grade dicalcium phosphate, or both.A representative process of the general type is described in copendingLe Baron application Serial No. 312,519, entitled Process for ProducingPhosphate Materials, now Patent No. 2,722,472, the disclosure of whichis incorporated herein by reference.

In such a process, sulfuric acid is a major item of expense and issubstantially entirely converted to calcium sulfate, a worthlessbyproduct which is discarded. The invention accordingly envisions in oneembodiment the use of an amount of sulfuric acid substantially less thanthat required to convert entirely to phosphoric acid the phosphatevalues of the processed phosphate rock.

Theoretically, the phosphate values of phosphate rock can bewater-solubilized by reaction of the rock with an amount of sulfuricacid requisite only to convert such phosphate values to monocalciumphosphate. The produotion of ordinary superphosphate entails such aprocedure in which the sulfuric acid may be employed in some cases in anamount up to about 10% in excess of that required to convert tomonocalcium phosphate the phosphate values of the rock. Unfortunately,ordinary superphosphate manufacturing processes yield a product in whichthe calcium sulfate is present in a form exceedingly diificult toseparate from an aqueous slurry of such product, such as that formed inan attempt to extract the monocalcium phosphate values thereof withwater.

The present invention accordingly contemplates, in the embodiment hereunder consideration, acidulating of particulate phosphate rock withsulfuric acid in an amount requisite to convert the phosphate values ofthe rock primarily to monocalcium phosphate, agitating or slurrying theacidulated rock with an aqueous medium, and filtering of the slurry soproduced, the entire procedure being carried'out under conditions and ina manner effective to produce calcium sulfate and other water-insolublematerials in a form more easily and readily separated from the liquidphase of the acidulated rock slurry.

Pursuant to this facet of the invention, particulate phosphate rock of aparticle size such that at least about 50% by weight, preferably about50% to about 85% by weight thereof, will pass through a 200 mesh screenis reacted with 60% to 70%, preferably 65% to 70%, aqueous sulfuric acidin an amount equal to from about 112% to about 117% of that required toform .monocalcium phosphate from the phosphatic materials contained inthe rock and to react with the impurities present therein. Theacidulated rock is then slurried or otherwise contacted, either beforeor after curing, with an aqueous medium to extract the monocalciumphosphate and other water-soluble phosphate values, and the slurry isprocessed to separate undissolved solids, primarily calcium sulfate, inconventional manner. It has been found that by following theseconditions, there is obtained a readily filterable slurry from which thecalcium sulfate and other acid-insoluble and water-insoluble impuritiesmay be readily and expeditiously removed.

Furthermore, the acidulated rock mix is produced in a form and conditionwhich can be readily processed and transported on a commercial basiswhich is not the case when the upper limit of acidulation of about 117%is substantially exceeded unless, of course, a sufi'icient amount ofsulfuric acid is employed to convert all of the phosphate values in therock substantially to orthophosphoric acid.

The following examples illustrate this feature of the invention:

Example 1 100 grams of particulate phosphate rock ground to a size suchthat about 50% to 60% thereof would pass a 200 mesh screen acidulatedwith about 65 aqueous su1 furic acid in an amount equal to 110% of thatrequisite to convert all of the phosphate values present in the rock tomonocalcium phosphate and to react with the impurities present therein.The acidulated rock so produced was cured for about one-half hour andthen slurried with water employed in an amount equal to 1.1 grams ofwater for each gram of acidulated rock. The slurry so produced washeated to 60" C. and filtered through a 11 cm. Buckner filter. Thefiltration rate was 13.6 gallons per hour per square foot.

Example 11 Example I was repeated with the exception that sulfuric acidwas employed in an amount equal to 115% of that requisite to convert allof the phosphate values in the rock to monocalcium phosphate and toreact with the impurities present. The acidulated rock mix was slurriedand filtered in the same manner as described in Example I. Thefiltration rate was 17.7 gallons per hour per square foot.

In the practice of this feature of the invention, at least during thetime the ground phosphate rock and sulfuric acid are admixed, intensiveand thorough agitation of the admixture is necessary. Although it isphysically possible to agitate this freshly prepared admixture for aconsiderable period of time, maximum recovery of phosphorus values intheir water-soluble forms is attainable when the period of agitation iskept as short as possible; provided, however, that the mixing issufliciently long and intense to afford intimate and uniformdistribution of sulfuric acid throughout the phosphate rock mass, andfurther provided the acidified rock is subsequently stored.

In a preferred embodiment of this feature of the invention, the slurry,once having been thoroughly mixed for a short time, generally not morethan a few minutes, is passed onto a continuous and moving belt on whichit is allowed to remain for a period of time, generally about 20 to 30minutes, sufricient to permit the soupy material to partially harden orset. The speed of the belt is such as to give a depth of materialsufficient to obtain a resultant set of the desired bulk density and toallow the required amount of time for the mix to only partially hardenor set, such that it is not soupy when discharged from the belt. Upondischarge from the belt, the material, having attained its initial set,is transferred to a storage pile. If it is deposited or piled prior tohaving reached its initial set stage, it is often diificult to removelater from the pile, so much so at times, that explosives may berequired to break it up. When handled as above described, however, it iseasily removed from the pile after storage for from five to fifteen daysby means of mechanical shovels or scoops, or manually. At all timesduring storage, and at the time of removal from storage, the material isporous and friable. The material remains in the storage pile to allowthe reactions to approach equilibrium and to bring the water-solublephosphorus pentoxide in the material up to the maximum, within practicallimits. Generally, a two-week storage time will result in a materialcontaining between about 94% and about 97% water-soluble phosphoruspentoxide, which is desirable at this point in the process, in view ofsubsequent process steps. The stored material is then easily pulped orslurried with an aqueous medium, since it is not set into a hard mass orlumps which require disintegration by application of explosives or useof harrunermills, etc. Sufficient aqueous medium, which may be water orpreviously produced extractor leach solution, is appropriately added sothat the initial resulting slurry contains between about 35% to about45% of undissolved solids although more concentrated or more diluteslurries can be formed. Agitation of the mixture of aqueous medium andacidulated phosphate rock results in an ultimate slurry containing aliquid phase in the form of a solution containing between about 20% and33% to 35% by weight of dissolved solids and about 67% to about byweight of water. The solid phase of the slurry takes the form ofundissolved solids which are discarded, which solids contain only about2.5% of the total phosphorus pentoxide values which were originallypresent in the rock and which consist primarily of calcium sulfate. Theliquid phase of the slurry, after separation from the undissolved solidphase, comprises essentially an aqueous solution of monocalciumphosphate and a small amount of phosphoric acid.

Separation of the solids from the extract solution so obtained may becarried out in any convenient and conventional manner such as, forexample, by filtration, countercurrent multistage decantation,preferably at about 5()60 C., by centrifuging or by use of liquid phasecyclone separators. Increasing the temperature increases the rate ofsettling or separation and, therefore, increases the capacity of thesettling or separation device. However, if material is held above 60 C.for any considerable length of time, some of the water-solublephosphorus pentoxide precipitates as insoluble dicalcium phosphate.

Aqueous solutions of phosphatic materials so produced are free ofcalcium sulfate and like materials, other than water, and have little orno value as animal feed supplements or fertilizers.

A threshold problem incident to the production of feed grade dicalciumphosphates from such extracts centers around the reduction of thefluorine content thereof to a degree requisite to the production of feedgrade dicalcium phosphate which has an elemental phosphorus to elementalfluorine weight ratio of not less than about 100, preferably not lessthan about 200. That aspect of this invention which relates to theproduction from acidulated phosphate rock of a substantially calciumsulfate free extract has particular relevance to feed grade dicalciumphosphate. Efforts to defluorinate extracts of acidulated phosphate rockin the presence of acid-insoluble materials,

including calcium sulfate, are commercially irfeasible and result inexcessive loss of phosphorus pentoxide values in the calcium sulfatewaste product. Accordingly, it is important to remove substantially allof the calcium sulfate from the acidulated rock extract prior to thedefluorination thereof.

The prior art has suggested e.g., defiuorination of solutions offluorine-containing phosphates, such as acidulated phosphate rockextracts, by addition thereto, inter alia, of alkali metal ions,preferably in the form of solutions of alkali metal salts, such assodium and potassium chlorides and the like. Such alkali metaldefluorination processes are ineffective to produce a defluorinated.extract or solution from which a dicalcium phosphate product of animalfeed grade can be produced. The addition of alkali metal ions to suchsolutions is effective only to increase the ole mental phosphorus toelemental fluorine weight ratio to a figure substantially below 100,normally less than about 50. The addition of excessive amounts of suchions results in a decrease in the phosphorus to fluorine elementalweight ratio in the extract due to the fact that more soluble forms ofalkali metal fluoride-containing compounds are produced.

Reference is made to FIGURE 1 which demonstrates that the addition ofsodium hydroxide to wet process 26% orthophosphoric acid obtained by theacidulation of phosphate rock with sulfuric acid in an amount sufficientto convert substantially all of the phosphorus values to orthophosphoricacid, followed by the extraction of the acidulated rock with water andthe removal of calcium sulfate, all in conventional manner, does notachieve substantial defluorination of such acid. It will be observedthat the sodium ions so introduced are effective to raise the elementalphosphorus to elemental fluorine weight ratio of the acid only to about20, which figure is reached after sodium ions have been added in anamount equal to about 200% of that requisite to form sodiumsilicofluoride with the fluorine present in the acid. It will beobserved that as additional amounts of sodium ions are added, theelemental phosphorus to elemental fluorine weight ratio again decreasesto less than 15. If excessive amounts of sodium ions are added, forexample, in an amount requisite to convert the phosphate present in thesystem from phosphoric acid to monosodium phosphate, the elementalphosphorus to elemental fluorine weight ratio of the resulting systemwill be substantially lower than required for satisfactory calciumdefluorination.

Calcium ion supplying compounds, such as inorganic oxygen-containingbasic calcium compounds, including calcium oxide, calcium hydroxide, andcalcium carbonate, can be employed to effect the requisitedefluorination of aqueous solutions of fluorine-containing phosphaticmaterials. Such calcium bases react with fluorine and other impuritiespresent, including aluminum and iron, to form fluorine-rich precipitateswhich can be separated as by filtration, decantation, or the like, fromthe defluorinated liquor in which the precipitate is formed.

Effective calcium defluorination of aqueous solutions offluorine-containing phosphatic materials through utilization solely ofcalcium bases results in the concurrent precipitation and loss ofexcessive amounts of phosphorus values to a fluorine-rich precipitatewhich is formed. Accordingly, one feature of this invention is addressedto a process for minimizing the loss of phosphorus pentoxide values tothe fluorine-rich precipitate formed when calcium bases or other formsof calcium ions, such as those derived from calcium oxide, calciumhydroxide, and calcium carbonate and the like, are employed todefluorinate aqueous solutions of fluorine-containing phosphaticmaterials, and particularly extracts or such solutions derived fromacidulated phosphate rock.

The quantity of phosphorus pentoxide lost to the fluorine-richprecipitate formed in the calcium defluorination of aqueous extracts,such as those obtained from acidulated phosphate rock, to produce adefluorinated extract having a predetermined minimum fluorine contenthas, to various workers in the art, heretofore been considered to besubstantially independent of the initial fluorine content of the extractprocessed. It has now been discovered that the calcium defluorination ofaqueous solutions of fluorine-containing phosphatic materials isattended by many of the characteristics of systems in which the solidphase or phases are not definite chemical compounds. It can be theorizedthat such calcium defluorination procedures result in the formation offluorine-containing solids crystallized with calcium phosphates in aform analogous to solid solutions of varying composition. The foregoingtheory is offered in the interest of completeness and is not limiting ofor definitive of this aspect of the invention.

In any event, in accordance with this feature of the invention, it hasbeen discovered that the loss of phosphate values to the defluorinationprecipitate in the calcium defluorination of aqueous solutions offluorine-containing phosphat-ic materials can be minimized bycontrolling or adjusting the elemental phosphorus to elemental fluorineWeight ratio of the solution to be defluorinated to a value of not lessthan about 15, and preferably to a value within the range of from about25 to about 50, prior to the calcium defluorination step. In general,two basic methods are available for the adjustment or control of theelemental phosphorus to elemental fluorine weight ratio in theacidulated rock extracts or other aqueous solutions offluorinerich-cont-aining materials which are to be defluorinatedpursuant to the invention.

Pursuant to the first mode of such control, a substantial amount offluorine may be removed from the acidulated rock prior to extractionthereof with an aqueous medium whereby an extract of controlled fluorinecontent is obtained in the first instance. For example, the elementalphosphorus to elemental fluorine weight ratio of an acidulated phosphaterock extract may be controlled to the desired value of not less thanabout 15 by curing or aging the acidulated rock mix under atmosphericconditions for a period of at least several days, normally at leastabout 5 days, and preferably at least about 14 days or longer,

' prior to extraction thereof with an aqueous medium. It

has been discovered that a sufficient amount of fluorine is eliminatedfrom the acidulated rock during such aging period to produce an ultimateextract having a phosphorus to florine weight ratio of not less thanabout 15. It will be appreciated that expedicnts can be availed of, forexample, to shorten the aging time, such expedients taking the form ofheating the acidulated rock mixture, or other methods known to the artto expedite chemical reactions.

Alternatively, there may be produced an extract from freshly acidulatedrock, which extract may be thereafter partially defluorniated by meansother than calcium ions prior to the calcium defluorination procedure.For example, alkali metal ions, such as those derived from alkali metalhydroxides, chlorides, sulfates, or the like, including sodium,potassium, and lithium chlorides, bromides, sulfates, hydroxides, andthe like, can be employed to increase the elemental phosphorus toelemental fluorine ratio of such extracts to a figure requisite to thefeasible calcium defluorination thereof. Accordingly, an aqueous extractof acidulated phosphate rock can be initially partially defluorinated bythe treatment thereof with the requisite amount of an alkali metal ion,such as an alkali metal hydroxide or halide, to produce a partiallydefluoririated solution having an elemental phosphorus to elementalfluorine weight ratio of not less than about 15 The defluorination canthereafter be completed by treatment of the partially defluorinatedextract with an appropriate amount of a calcium base material, such ascalcium carbonate, calcium oxide, or calcium hydroxide, to produce anultimate defluorinated extract in which the elemental phosphorus toelemental fluorine weight ratio is at least about 100.

The particular degree of acidulation of the rock from which the aqueousextracts of fluorine-containing phos- 7 phatic materials are derived isnot a critical feature of this aspect of the invention. Phosphate rockcan be acidulated to any desired degree with any desired mineral acid,such as sulfuric acid, phosphoric acid, hydrochloric acid, or nitricacid, effective to convert the phosphate values in the rock towater-soluble form. It is contemplated, of course, that the acidulatingacid will be employed in an amount suflicient to convert the predominantamount of the phosphate values present in the rock to water-solublephosphate compounds, such as monocalcium phosphate or phosphoric acid,or mixtures thereof. The acidulation process described in the precedingportions hereof can appropriately be employed to produce suitablemonocalcium phosphate solutions.

The use broadly of calcium ions to defluorinate aqueous solutions offluorine-containing phosphatic materials is known to the art. Such priorart procedures can be employed in the calcium defluorination step ofthis feature of the invention. In general, such procedures entail theaddition of inorganic calcium bases, such as calcium oxide, calciumhydroxide, or calcium carbonate, preferably in the form of a slurry, tothe solution of phosphatic mateiial to be defluorinated. Reference ismade to Le Baron application Serial No. 424,712 entitled Method ofPreparing Defluorinated Material, now Patent No. 2,889,200, whichdescribes a preferred method for the calcium defluorination of aqueousextracts of acidulated phosphate rock. In accordance with the teachingsof that application, such extracts are first adjusted to a phosphoruspentoxide concentration in the range of about 11% to about by weight andthen reacted with calcium carbonate in an amount sufiicient to produce apH in the resulting reaction mixture between about 2.3 and 3.0, thereaction time being generally from about to about 120 minutes. Thisinvention, of course, is not restricted to the specific defluorinationprocedure disclosed in the aforementioned Le Baron application but isembracive of calcium defluorination operations generically of aqueoussolutions of fluorine-containing phosphatic materials in which theelemental phosphorus to elemental fluorine weight ratio is controlled,as contemplated by the invention prior to the calcium defluorinationstep.

Reference is made to FIGURE 2 which shows the percentage of phosphatevalues retained in the defiuorinated extract in the calciumdefluorination of an aqueous extract of acidulated rock with bothcalcium oxide and calcium carbonate as a function of the elementalphosphorus to elemental fluorine weight ratios in the treated extracts.The data represented in the figure are those resulting from the calciumdefluorination of the extracts in question to produce in thedefluorinated extract an elemental phosphorus to elemental fluorineweight ratio of 200. The extract solutions employed were wet processphosphoric acid produced by the sulfuric acidulation of phosphate rockfollowed by agitation of the acidulated rock with water and removal ofcalcium sulfate from the slurry so formed, all in conventional mannerwell known to the art, as described, for example, in the book PhosphoricAcid, etc. by Waggaman, chapter 12, 2nd edition. The calcium oxide andcalcium carbonate were added in the form of an aqueous slurry. Theextract which was defluorinated was adjusted to a phosphorus pentoxidecontent of about 18% by weight and the calcium bases were added in anamount sufficient to produce a pH in the reaction mixture between about2.3 and 3.0. The contact time between the calcium bases and the extractto be defluorinated was on the order of about 30 to minutes.

It will be apparent from an examination of FIGURE 2 that the quantity ofdicalcium phosphate retained in the defluorinated extract increasessubstantially as the ratio of elemental phosphorus to elemental fluorinein the extract to be defiuorinated is increased. Byreference to thecurve, it will be observed that the untreated wet process phosphoricacid was characterized by an elemental phosphorus to elemental fluorineweight ratio of about 6. Calcium defluorination of such untreated acidresulted in a loss of more than 55% of the phosphorus pentoxide valuesto the fluorine-rich precipitate. Adjustment of the elemental phosphorusto elemental fluorine weight ratio in the extract prior to calciumdefluorination to a value of at least 15 made it possible to retain inthe defluorinated extract more than 70% of the phosphorus pentoxidevalues. The control of the elemental phosphorus to elemental fluorineweight ratio in the extracts employed to obtain the data graphicallyreflected in FIGURE 2 was effected by the addition of alkali metal ionsto the extract prior to the calcium defluorination in the case of thehigher elemental phosphorus to element fluorine weight ratioadjustments, and in some cases, by aging the acidulated rock from whichthe extract was obtained in the case of the lower of such ratios.

The invention further relates to the production from aqueous extracts ofacidulated phosphate rock of high analysis phosphate fertilizerscontaining available phosphorus pentoxide in both water-soluble andwater-insoluble form. As in the case of feed grade dicalcium phosphate,it is essential in the conversion of such extract solutions to highanalysis phosphate fertilizers, that the waste product calcium sulfatebe effectively and efficiently separated therefrom. It is preferred toproduce such calcium sulfate free extracts by the procedure heretoforedescribed wherein the phosphate values of phosphate rock are extractedprimarily in the form of monocalcium phosphate. This facet of theinvention, however, in at least some of its ramifications, extends tothe production of high analysis fertilizers from alternative types ofaqueous acidul'ated phosphate rock extracts, as well as from aqueoussolutions or slurries of phosphate materials derived from sources otherthan phosphate rock.

Aqueous extracts of acidulated rock, including such extracts in whichthe phosphate values are present predominantly as monocalcium phosphate,do not yield a satisfactory commercial fertilizer product merely bydehydration. Commercially acceptable fertilizer products can beobtained, however, through addition to such extracts of calcium bases,such as calcium oxide, calcium carbonate, and calcium hydroxide,provided the extracts to which such bases are added are properly andeffectively dehydrated to produce a granular or pellet type fertilizer.

It is generally preferred to employ such calcium bases in an amountrequisite to provide in the resulting mixture a calcium oxide tophosphorus pentoxide mole ratio of at least about 0.85, and preferablyat least about 1.0. The upper limit of calcium base addition is afunction of the amount of water-insoluble phosphorus pentoxide desiredin the form of dicalcium phosphate in the final product. An appropriateupper limit for the amount of calcium base material is in an amountrequisite to provide in the resulting reaction mixture a calcium oxideto phosphorus pentoxide mole ratio of not more than about 1.2. Asuitable range for such calcium base addition is an amount of calciumbase requisite to produce in the extract to which the base is added acalcium oxide to phosphorus pentoxide mole ratio of from about 1.1 toabout 1.4. Extracts to which calcium bases have been added in the amountindicated, if properly dried, yield a fertilizer product which isnon-hygroscopic and which contains both water-soluble andwater-insoluble forms of available phosphorus pentoxide, containing fromabout 45% to about 55 by weight of phosphorus pentoxi'de of which about20% to about 40% is water-insoluble.

The proper dehydration of calcium base modified acidulated phosphaterock extracts entails a procedure whereby there is produced from suchcalcium treated extracts a high analysis fertilizer in the form ofgenerally smoothsurfaced, porous pellets comprising a plurality ofgenerally concentric layers of phosphate fertilizer material extendingfrom the center of said pellets outwardly, said layers being ofprogressively increasing diameter. The

mode of drying is such that there is produced in the final pelletizedfertilizer product from about to about 15% by weight of calciumpyrophosphate and from about 2% to about by weight of calciummetaphosphate.

Reference is made to FIGURE 3 which shows schematically the structure ofthe pelletized fertilizer product of this invention. The schematicdrawing represents a pellet of the fertilizer product of this invention,one end of which has been removed as by cutting to show a cross sectionthereof. It will be observed that there are a plurality of generallyconcentric layers of fertilizer material extending from the center ofthe pellet outwardly. It may be expected that calcium pyrophosphate andcalcium metaphosphate, which form a part of the fertilizer prodnot ofthe invention, may be positioned or concentrated at the outer surfacesof the various layers, and particularly on the outer surface of thepellet when the pellets are prepared by drying the calcium base treatedextracts in the desired manner, as hereinafter described.

In accordance with this aspect of the invention, there is provided acirculating bed of nuclei about which the pellets of fertilizer productare formed. Such nuclei preferably take the form of solid particlesobtained by drying a calcium base treated aqueous extract of acidulatedrock. Other pellet nuclei, such as particularly phosphate rock, sand,and the like, can be employed, particularly at the initiation of thepellet-forming process. Such circulating bed of pellet nuclei isrepeatedly circulated under a spray of calcium base treated aqueousextract of acidulated rock and dried until there is produced a phosphatefertilizer in the form of a generally smooth-surfaced, porous, pelletcomprising a plurality of generally concentric layers of fertilizermaterial extending from the center of said pellet outwardly and being ofprogressively increasing diameter. under conditions such that thetemperature of the pellets does not at any time exceed a temperature ofabout 200 C., and the final moisture content is not in excess of about5%, preferably from about 1% to about 5%.

In the preferred practice of the invention, the bed of pellet nuclei iscirculated through a direct fired rotary drier, and calcium base treatedacidulated phosphate rock extract is continuously sprayed on therecirculating load within the drier. The drier is maintained at atemperature of about 150 C. to about 250 C., and preferably about 175C., with precautions being taken to insure that the temperature of theproduct does not exceed about 200 C. and to produce a final producthaving a moisture content of not more than about 5%, preferably fromabout 1% to about 5%. It will be appreciated? that instead of sprayingthe circulating load of solids with the calcium base treated extractinside the drier, such solids may be sprayed outside the drier andthereafter intro duced into the drier as a wet feed.

In the preferred practice of this feature of the invention,approximately one part by weight of the calcium base treated aqueousextract of acidulated rock is added to each 4 to 10 parts by weight ofrecirculating solids, preferably the addition of calcium base treatedextract to the solids is adjusted so that the mixture in the drier doesnot contain at any time more than about by weight of moisture. Theproduct discharged from the drier is screened for suitable mesh size,and is granular, non-dusting, and non-hygroscopic. The form of drierused does not constitute a critical feature of the invention; driersother than rotary driers can be employed.

An appropriate calcium base material for use in the production of highanalysis phosphate fertilizers of the invention takes the form of thefluorine-rich precipitate which is obtained in the defluorinationreaction previously described with reference to the production ofdicalcium phosphate. Such a fluorine-rich precipitate can beappropriately reacted with an extract obtained from acidulated phosphaterock. Such extracts may be obtained from either cured or uncuredacidulated phosphate rockv A The drying of the pellets is carried outparticularly suitable extract is one obtained by acidulating phosphaterock in the manner previously described with an amount of sulfuric acidequal to from about 112% to about 117% of that required to convert thephosphate values present in the rock to monooalcium phosphate, andthereafter without any substantial storage time, leaching or otherwiseextracting the solubilized phosphate values from the rock to produce a.green or unaged extract. Normally such an extract can be prepared byprocessing the acidulated phosphate rock within a few minutes, normallywithin about 15 to about 90 minutes, after the acid and rock have beencombined. The resulting extract is rich in phosphoric acid and may beexpected to be characterized by a calcium oxide to phosphorus pentoxidemole ratio of from. about 0.18 to about 0.30. Other types of aqueousextracts of acidulated phosphate rock can, of course, be employed.

The fluorine-rich precipitate produced incident to the production ofdicalcium phosphate, as above described, can be reacted with such agreen extract in an amount requisite to provide the desired calciumoxide to phosphorus pentoxide mole ratio in the ultimate fertilizerproduct. The reaction mixture or slurry so produced is formed into apelletized fertilizer product in the same manner as previous-1ydescribed to produce a multilayered, pelletized, high analysis phosphatefertilizer con taining from about 45% to about 60% by weight ofavailable phosphorus pentoxide. Such a product contains from about 30%to about 50% by weight of monocalcium phosphate, from about 5% to about15% by weight of dicalcium phosphate, from about 0.5% toabout 2% byweight of tricalcium phosphate, from about 2% to about 10% by weight ofcalcium. metaphosphate, from about 5% to about 10% of calciumpyrophosphate, from about 3% to about 8% by weight of free phosphoricacid, and minor percentages of fluorine, aluminum, and iron containingmaterials aggregating not more than about 10% by weight of thefertilizer product.

The use of the fluorine-rich precipitate formed in the manufacture ofdicalcium phosphate in the production of a high analysis phosphatefertilizer product is of particular significance in that embodiment ofthe invention which contemplates an integrated process for thesimultaneous or concurrent production of high analysis fertilizers andfeed grade dicalcium phosphate. Such a process is described in greaterdetail in the aforementioned copending Le Baron application Serial No.312,519.

A specific embodiment of the invention, as addressed to the productionfrom phosphate rock of a granular nonhygroscopic, free-flowingfertilizer containing a high percentage of phosphorus pentoxide which issubstantially completely available and which is present in bothwatersoluble and water-insoluble forms, entails (1) continuously mixingand agitating phosphate rock ground to a particle size such that about50% to about thereof will pass through a 200 mesh screen with 65% to 70%aqueuos sulfuric acid, said sulfuric acid being employed in an amountequal to about 112% to about 117 preferably about 1 15 of that requiredto form monocalcium phosphate from the phosphatic materials contained insaid rock and to react with the impurities contained in said rock, saidmixture being agitated for not more than about 4 minutes, preferably notmore than about 1 minute; (2) discharging the agitated mixture onto acontinuously moving conveyor and maintaining said mixture on saidconveyor for about 20 to 30 minutes to permit said mixture to initiallyset; thereafter discharging said mixture in initially set condition ontoa curing pile and storing said mixture in said curing pile for severaldays, preferably about 5 to 15 days, to produce a cured phosphatematerial which is friable and porous and which, without prior mechanicaldisintegration, is easily slurried with an aqueous medium; (3) formingan initial aqueous slurry of said cured phosphate material containingabout 35% to about 40% solids; (4) agitating said slurry for a shortperiod of time to form a liquid phase containing about 20% to about 33%dissolved solids and about 67% to about 80% water; (5) heating saidslurry to an elevated temperature not in excess of 60 C. and separatingsaid liquid phase from the non-dissolved solids materials contained insaid slurry and adding to said separated liquid phase a small amount ofan inorganic calcium base, such as limestone; (6) thereafter introducingsaid liquid phase into a load, recirculating through a drier, of aparticulate solid phosphate material resulting from the priordehydration of similarly obtained liquid phase compositions, said liquidphase being added to said recirculating load in an amount such that theresulting combination does not contain more than about 15% of moisture,there being employed one part of said liquid phase for each 4 to partsof solids in said load; (7) introducing said combination into said drier'w-hile said drier is operated at a temperature of about 150 C. to about230 C. and maintaining said combination in said drier for a time periodsufiicient to efiect reduction of the moisture content thereof tobetween about 1% and about 5% by weight and to produce at a temperaturenot in excess of 200 C. a granular, non-hygroscopic, free-flowingproduct containing about 55% to about 58% phosphorous pentoxide, ofwhich about 20% to about 40%, preferably about 35%, by weight iswater-insoluble, said available phosphorus pentoxide constitutingsubstantially the entire amount of phosphorus pentoxide present in saidproduct.

It will be appreciated that the lacidulated phosphate rock may be simplydischarged from the acidulating vessel into a cured or storage pilewithout necessarily being transported to such pile by means of a movingconveyor, although it is desirable to the end that a friable, curedproduct may be obtained, to permit the mixture to initially set and thendisturb the initially set condition. Further, the specific embodiment ofthe invention, as described in detail above, may be practiced with anextract of freshly acidulated rock rather than an extract of cured rock.

In the practice of this invention for the production of a most desirableform of acidulated rock mixes, the rock is ground to a particle sizesuch that substantial-1y the entire amount of the rock will pass a 14mesh screen with about 50% to about 80% by weight being of a particlesize requisite to pass a 200 mesh screen.

The following examples are illustrative of the best modes presentlyknown to the applicants for practicing the various features of theinvention.

Example III About 10 tons per hour of Florida phosphate rock was groundto a particle size approximately 52% of which passed through a 200 meshstandard screen. This rock analyzed about 67% bone phosphate of lime.The ground rock was mixed with about 6 tons per hour of about 98%sulfuric acid added as 51 to 54 B. aqueous solution. The mixture wasthoroughly agitated for about two minutes after which it was dischargedonto a continuous belt provided with exhaust means for gases such assulfur dioxide, silicon tetrafluoride and the like, placed adjacent thepoint of discharge of slurry onto the belt. The belt length and itsspeed were such that the mixture remained on the belt about 20 minutes.The discharge from the belt was stored in a pile for about 14 days.

The stored material was then removed from storage, broken up andsuflicient water added to give a slurry of about 35 undissolved solids.The slurry was subjected to four steps of continuous countercurrentdecantation followed by a single filtration to produce a leach solutioncontaining about 30% dissolved solids being of approximately 32 B.gravity. The discarded tailings contained about 2.5% of the 30% totalphosphorus pentoxide, only about one-half of which is available. Thisextract was delivered to the storage tank 37, and had an elementalweight P/F of about 20'. I

A second acid mix was prepared utilizing the same proportions, but wasnot sent to storage. The green or unaged mix was delivered directly tothe countercurrent decantation operation to produce a leach solutioncontaining approximately the same percentage of dissolved solids as theextract from the aged superphosphate. This solution was delivereddirectly to storage tank 38. The extract from aged superphosphate wasfurther processed by adding thereto approximately 6 parts by weight oflimestone per parts by weight of extract solution. It will be recognizedthat other materials capable of reacting in the same molecularproportions may be substituted for limestone, such as cmcium oxide. Themixing of these proportions of ingredients results in the precipitationof the major portion of the fluoride present in the extract to produce afiltrate having approximately 15 phosphorus pentoxide, 0.04% fluorine,and a calcium oxide to phosphor-us pentoxide ratio of approximately 0.9.To this filtrate is added approximately 12 parts by weight of comminutedlimestone per 100 parts of defluorinated extract. The result of thereaction of these ingredients is the precipitation of a material whichis predominantly dicalcium phosphate. The dicalcium phosphate solids arefiltered ofi and show a recovery of approximately 99% of the phosphatespresent in the ex: tract solution. The dicalcium phosphate is dried andthe product contains approximately 20% phosphorus, and an elementalweight P/F in excess of 100.

The filter cake containing the precipitated fluorides was mixed withextract from green superphosphate from storage tank 38 in theproportions of approximately 11 pounds of cake per 100 pounds by weightof green extract solution. The resultant slurry is dried in a rotarykiln and screened to produce a 3+l2 mesh product containingapproximately 56% available phosphorus pentoxide, the drying beingeffected as in Example II.

Example IV About 10 tons per hour of phosphate rock, ground to 50%passing through a 200 mesh screen and of about 67% bone phosphate oflime analysis, was mixed with about 6 tons per hour of about 96%sulfuric acid added as 50-60 B. aqueous solution. The mixture wasthoroughly agitated for about one minute, after which it was dischargedonto a continuous belt. The belt length and its speed were such that themixture remained on the belt about twenty minutes. The discharge fromthe belt was stored in a pile for about fourteen days. The storedmaterial was then removed from storage, broken up, and sufficientsolution from the process (previously prepared leach solution plus addedwater) added to give a slurry of about 35% non-dissolved solids.Continuous multistage countercurrent decantation (for example, using atray washer), produces a leach solution containing about 30% dissolvedsolids. This is the primary leach or phosphate solution. This solutionis freed of non-dissolved solids by centrifuging, filtering, or by theuse of cyclone separators, and has a P/F weight ratio of about It isthen further processed by adding about 0.45 ton per hour of marble flouror other suitable limestone to the leach solution. and granulated in adirect fired rotary kiln at a temperature not exceeding about 200 C. Inthe drying of the product there was provided within the rotary kiln arecirculating bed of previously dried material such that there was builtup a pelletized product in the form of a series of layers of highanalysis phosphate fertilizer extending from the center of said pelletsoutwardly. The bed of material which was recirculated through the driedwas continuously screened and a desired portion of properly sizedpellets separated therefrom. Undersized material was recirculated andoversized material was ground and recirculated as a part of such bed.The final pelletized product contained from about 30% to about 50% byWeight of monocalcium phospate, from about The solution is continuouslydried' to about 15% by weight of dicalcium phosphate, from about 0.5% toabout 2% by weight of tricalcium phosphate, from about 2% to about byweight of calcium metaphosphate, from about 5% to about 10% by weight ofcalcium pyrophosphate, and from about 3% to about 8% by weight of freephosphoric acid. The product was non-hygroscopic and contained fromabout 55% to about 56% by weight of available phosphorus pentoxide ofwhich about 30% to about 35% by weight was water soluble.

In place of using about 0.45 ton of marble flour, about 0.25 ton ofcalcium oxide, or its equivalent in calcium hydroxide alternatively, maybe used. This product also contains about 55-5 6% phosphorus pentoxidein available form. Also, in producing this product (i.e., the 55 56%available phosphorus pentoxide) substantially equivalent molarquantities of calcium oxide or calcium hydroxide, as compared with themolar quantities of marble flour, can be employed, Dolomitic limestoneor other similar limestone may be employed in place of marble flour. Inthis instance, the final product will be found to contain a lowerfluorine content than when ordinary limestone is used. Mixtures ofquicklirne or hydrated lime with limestone are also suitable for use.

Example V An extract of green or unaged acidulated phosphate rock wasprepared in the same manner as described in Example HI, particularlywith reference to the treatment of the second acid mix there referredto. The extract so obtained was characterized by an elemental phosphorus to elemental fluorine weight ratio of about 6. The extract soproduced was treated by contact thereof with superheated steam in amanner generally similar to that described in Patent No. 2,165,000 toincrease the elemental phosphorus to elemental fluorine mole ratio to avalue of about 25. The extract was then reacted with calcium carbonatein an amount requisite only to substantially completely precipitate thefluorine-containing materials present therein. The fluorineprecipitation reaction was carried out at a pH of about 2.5 for a timeperiod of about 45 minutes and the precipitate so formed separated fromthe mother liquor. About 85% of the phosphate values originally presentin the extract were retained in the mother liquor which was thereafterreacted in conventional manner with additional calcium carbonate toproduce a feed grade dicalcium phosphate precipitate which was recoveredand dried.

In lieu of steam, alkali metal ions, for example in the form of aqueoussolutions of alkali metal salts, such as sodium and potassium chloride,sulfate, and the like, can be employed to raise the elemental phosphorusto elemental fluorine weight ratio of such extracts to a value of notless than 15.

The term feed grade dicalcium phosphate is employed herein as embrasiveof dicalcium phosphates having a fluorine content sufiiciently low to beacceptable for use as animal feed supplements.

This application is a division of Manning and Le Baron applicationSerial No. 511,624, filed May 27, 1955, which is a continuation-in-partof Manning and Le Baron application Serial No. 151,728, now abandoned,the disclosure of which is incorporated herein by reference. Relatedinventions are disclosed in Le Baron applications 14 Serial Nos.186,850, now Patent No. 2,709,649, and 311,950, now Patent No.2,761,775.

We claim:

A process for producing from phosphate rock, a granular,non-hygroscopic, free-flowing fertilizer containing a high percentage ofphosphorus pentoxide which is substantially completely available, saidphosphorus pentoxide being present in both water-soluble andwater-insoluble form and, hence, available to plants both immediatelyand also throughout the various stages of growth, which process consistsessentially of: 1) continuously mixing and agitating phosphate rockground to a particle size such that about 50% to about 85% thereof willpass through a 200 mesh screen with to aqueous sulfuric acid, saidsulfuric acid being employed in an amount equal to about of thatrequired to form monocalcium phosphate from the phosphatic materialcontained in said rock and to react with the impurities contained insaid rock, said mixture being agitated for not more than about 4minutes; (2) discharging the agitated mixture onto a continuously movingconveyor and maintaining said mixture on said conveyor for about 20 to30 minutes to permit said mixture to initially set; thereafterdischarging said mixture in initially set condition onto a curing pileand storing said mixture in said curing pile for about 5 to 15 days toproduce a cured phosphate material which is friable and porous andwhich, Without prior mechanical disintegration, is easily slurried withan aqueous medium; (3) forming an initial aqueous slurry of said curedphosphate material containing about 35% to about 40% solids; (4)agitating said slurry for a short period of time to form a liquid phasecontaining about 20% to about 33% dissolved solids and about 67% toabout 80% water; (5) heating said slurry to an elevated temperature notin excess of 60 C. and separating said liquid phase from thenon-dissolved solids materials contained in said slurry and adding tosaid separated liquid phase a small amount of limestone; (6) thereafterintroducing said liquid phase into a load, recirculating through adrier, of particulate solid phosphate material resulting from the priordehydration of similarly obtained liquid phase compositions, said liquidphase being added to said recirculating load in an amount such that theresulting combination does not contain more than about 15% of moisture,there being employed one part of said liquid phase for each 4 to 10parts of solids in said load; (7) introducing said combination into saiddried While said drier is operated at a temperature of about C. to about230 C. and maintaining said combination in said dried for a time periodsufficient to effect reduction of the moisture content thereof tobetween about 1% and about 5% by weight and to produce at a temperaturenot in excess of 200 C. a granular, non-hygroscopic, free-flowingproduct containing about 55 to about 58% phosphorus peutoxide, of whichabout 35 by weight is water-in soluble, said available phosphoruspentoxide constituting substantially the entire amount of phosphoruspentoxide present in said product.

References Cited in the file of this patent UNITED STATES PATENTS2,709,649 Le Baron May 31, 1955

